The present invention relates to a dry radioactive substance storage facility and, more particularly, to a radioactive substance storage facility for storing radioactive substances including high-level radioactive wastes and radioactive substances produced in nuclear power plants.
Aggregate of radioactive substances used as fuel assemblies in a nuclear power plant is reprocessed to recover fissionable nuclear fuel substances including uranium and plutonium. High-level radioactive wastes resulting from the reprocessing of radioactive substances are vitrified. Since the vitrification of radioactive wastes generates decay heat, the vitrified radioactive wastes must be stored and cooled until the heat generation rate of the radioactive wastes decreases to an extent that permits disposal. Aggregate of radioactive wastes is kept in a storage pool in the nuclear power plant. The amount of aggregates of radioactive substances increases year by year and, eventually, exceeds the capacity of the storage pool of the nuclear power plant. Therefore, it is desired to construct a new storage facility capable of storing a large amount of aggregate of radioactive substances for a long time.
The storage facility disclosed in Japanese Patent Laid-open No. Hei 8-43591 relates to a shielding structure.
A dry radioactive substance storage facility stores vitrified radioactive wastes and aggregate of radioactive substances while cooling the same by air that flows around the same. Dry radioactive substance storage facilities are disclosed in Japanese Patent Laid-open Nos. Hei 5-273392 and Hei 8-43591.
The dry radioactive substance storage facility disclosed in Japanese Patent Laid-open No. Hei 5-273392 has a concrete building having partition walls of a radiation shielding substance and defining a storage room. Many storage tubes are placed in the storage room and aggregates of radioactive substances are contained in the storage tubes. Decay heat generated by the radioactive substances contained in the storage tubes is dissipated into cooling air for cooling. The cooling air flows horizontally through spaces between the storage tubes in a cooling air passage between the ceiling slab and the floor slab of the storage room.
The dry radioactive substance storage facility disclosed in Japanese Patent Laid-open No. Hei 8-43591 is characterized by the use of a heat-resistant resin for forming a structure for enclosing radioactive substances therein.
The dry radioactive substance storage facility disclosed in Japanese Patent Laid-open No. Hei 5-273392 has walls capable of shielding radioactive rays emitted by radioactive substances. However, nothing is mentioned in Japanese Patent Laid-open No. Hei 5-273393 about the improvement of radiation shielding ability.
In the dry radioactive substance storage facility disclosed in Japanese Patent Laid-open No. Hei 8-43591, some of structural members are formed of a substance capable of shielding radioactive rays. Therefore, the number of structures only for radioactive ray interception can be attenuate. However, nothing is mentioned in Japanese Patent Laid-open No. Hei 8-43591 about cooling ability.
In those prior art dry radioactive substance storage facilities, structure is discussed only on the basis of examination of cooling structure or shielding ability, and nothing is discussed on the interception of radioactive rays.
Accordingly, it is an object of the present invention to provide a dry radioactive substance storage facility capable of improving radioactive ray shielding ability without significantly reducing cooling performance.
With the foregoing object in view, the present invention provides a dry radioactive substance storage facility comprising a plurality of first radiation shielding members disposed in a storage room on the side of an air inlet duct through which air is supplied into the storage room, a plurality of second radiation shielding members disposed in the storage room on the side of an stack through which air is discharged outside, in which first air passages are formed between the first radiation shielding members, and second air passages are formed between the second radiation shielding members.
The air inlet duct can be shielded from radioactive rays by the first radiation shielding members, and the stack can be shielded from radioactive rays by the second radiation shielding members. Accordingly, the amount of radiation reaching the outside through the air inlet duct and the stack is attenuated greatly, whereby the radiation shielding performance of the dry radioactive substance storage facility is improved. Since air flows through spaces between the first and the second radiation shielding members, there is no possibility that the ability to cool the radioactive substance contained in the storage tubes is attenuated significantly.
Preferably, the first and the second radiation shielding members are arranged in a plurality of rows in the flowing direction of air, the first radiation shielding members in the row on the upstream side with respect to the flow of air are disposed opposite to spaces between the first radiation shielding members in the row on the downstream side with respect to the flow of air, and the second radiation shielding members in the row on the downstream side with respect to the flow of air are disposed opposite to spaces between the second radiation shielding members in the row on the upstream side with respect to the flow of air.
Since the first radiation shielding members in the row on the upstream side are disposed opposite to spaces between the first radiation shielding members in the row on the downstream side, the first radiation shielding members on the upstream side are able to intercept radiation through the spaces toward the air inlet duct. Since the second radiation shielding members in the row on the downstream side are disposed opposite to spaces between the second radiation shielding members in the row on the upstream side, the second radiation shielding members on the downstream side are able to intercept radiation propagating through the spaces toward the stack. Thus, the dry radioactive substance storage facility has a further improved radiation shielding ability.
Preferably, the first radiation shielding members are thermally connected to storage tubes on the upstream side of the storage tubes, and the second radiation shielding members are thermally connected to the storage tubes on the downstream side of the storage tubes. Since the radiation shielding members are thermally connected to the storage tubes, the radiation shielding members are cooled by air. Consequently, the reduction of cooling ability due to the partial covering of the storage tubes with the radiation shielding members can be avoided.
Preferably, a plurality of first baffling members are disposed on the side of the air inlet duct in the storage room, a plurality of second baffling members are disposed on the side of the stack in the storage room, and each of the first and the second baffling members comprises a neutron shielding member and a gamma ray shielding member. The first baffling members exercise the same function as the first radiation shielding members, and the second baffling members exercise the same function as the second radiation shielding members. The first and the second baffling members are capable of shielding neutrons and gamma rays.
Preferably, the first and the second baffling members are formed by covering a neutron shielding member with a gamma ray shielding member. Since the neutron shielding member is covered with the gamma ray shielding member, the gamma ray shielding member can easily be attached to the neutron shielding member.
Preferably, upper ones of the plurality of the second baffling members overlap more deeply and are dislocated more greatly toward the stack. Since upper ones of the plurality of the second baffling members overlap more deeply and are dislocated more greatly toward the stack, radiation propagating toward the stack are intercepted by the second baffling members, which enhances the effect of shielding radiation propagating toward the stack.
Preferably, the first baffling members are disposed in an inclined position so as to guide air supplied through the air inlet duct toward an upper region of the storage room. Since the first baffling members are disposed in an inclined position so as to guide air supplied through the air inlet duct toward an upper region of the storage room, the first baffling members utilizes a radiation attenuating function and a radiation reflecting function for shielding radiation. Consequently, the amount of radiation propagating toward the air inlet duct can greatly be attenuated. Part of radiation propagating toward the first baffling members is attenuated as the same permeates the first baffling members, and part of radiation is reflected downward by the inclined first baffling members. Consequently, the amount of radiation propagating toward the air inlet duct can greatly be attenuated.
Preferably, radiation scattering members are disposed in the storage room on the side of the air inlet duct relative to the first radiation shielding members, and on the side of the stack relative to the second radiation shielding members. The radiation scattering members scatters radiation propagated through the spaces between the adjacent baffling members to attenuate the amount of radiation propagating toward the air inlet duct and the stack.
Preferably, each of the radiation scattering members comprises a neutron shielding member and a gamma ray shielding member. Preferably, the radiation scattering member is formed by covering the neutron shielding member with the gamma ray shielding member. Preferably, a surface of the radiation scattering member facing the fuel storage room has a shape to reflect radiation downward.